There is known a hybrid vehicle including a power source such as an electromotor and a motor generator, in addition to an internal combustion engine. In the hybrid vehicle, while the internal combustion engine operates as efficiently as possible, excess and deficiency of a driving force and an engine braking force are adjusted by the electromotor and/or the motor generator.
As for the above hybrid vehicle, there is disclosed an example of a speed change mechanism capable of operating with switching an infinite variable speed mode and a fixed gear ratio mode in Patent Reference-1. Concretely, a power distribution mechanism including four revolution components is formed by combining two planetary gear mechanisms, and the four revolution components are connected to an engine, a first motor generator, an output axis and a brake, respectively. In such a state that the brake is released, the number of revolutions of the engine continuously changes by continuously changing the number of revolutions of the first motor generator, and the operation in the infinite variable speed mode is executed. Meanwhile, in such a state that the brake is fixed, the gear ratio is fixed by preventing the revolution of one of the above-mentioned revolution components, and the operation in the fixed gear ratio mode is executed. In addition, as the speed change mechanism for switching the infinite variable speed mode and the fixed gear ratio mode, there is known a speed change mechanism which applies not a normal wet multiple disc clutch but an engagement mechanism for engaging teeth of the revolution component and teeth of the fixed component.
In Patent Reference-2, there is disclosed a technique for a hybrid vehicle which calculates an engine torque based on a reaction force of a torque of an electromotor (first motor generator).
However, in the technique disclosed in Patent Reference-1, it is sometimes impossible to appropriately estimate the engine torque in such a state that teeth of the revolution component and teeth of the fixed component are engaging, i.e., in such a state that the fixed gear ratio mode is set. This is because, since the reaction torque of the engine is mechanically supported, not by the first motor generator, but by the engaging mechanism during traveling in the fixed gear ratio mode (in this case, the first motor generator outputs little torque), it is difficult to estimate the engine torque by using the torque of the first motor generator. There is not disclosed a method of estimating the engine torque with high accuracy during traveling in the fixed gear ratio mode in Patent Reference-2, too.
Patent Reference-1: Japanese Patent Application Laid-open under No. 2004-345527
Patent Reference-2: International Patent Application Laid-open under No. 2000/39444